This achievement requires not only The Dragon expansion, but also The City expansion! Since all Varthrax(red) deck dragons/enemies are Strength based, all Grilipus(green) deck dragons/enemies are Craft based, and only Cadorus(yellow) deck dragons/enemies can be a mix of either, it's impossible to trade one of each in at the same using the standard rules for trading in trophies.
So here's what you do:
1. Make sure you've activated both The City and The Dragon expansions for this game.
2. Choose an Alternative Ending that will have you encounter dragons of all three factions regularly. (I think this goes for all the endings for The Dragon expansion except for "Domain of Dragons". I got this on "Dragon Slayers", but other endings should also work just fine.)
3. Play as the Dragon Hunter.
4. Encounter and defeat one dragon from each faction(red, green, yellow), so that you get trophies from all of them. And make sure that the strength/craft for these trophies adds up to a total of 7 or higher.
5. Go to the Academy on the City board. (Accessible via the City space in the top left corner of the board.)
6. Choose to trade in trophies. Select the three you want to trade, and make sure they're one of each colour. If their strength/craft total adds up to 7 or more, you get the choice to trade them in for either +1 Strength or +1 Craft. Choose either, and end your turn.
You should now have the achievement!